In a short arc type discharge lamp that encloses mercury, a distance between the tips of a pair of electrodes, provided to face each other is short. A light source thereof is similar to a point source, so that such a lamp is combined with an optical system for use as a light source of an exposure apparatus with high light collection efficiency. Moreover, a short arc type discharge lamp, in which xenon is enclosed, is used as a visible light-light source of a film projector, and is also used as a light source of a digital cinema. Further, in such a short arc type discharge lamp, emitter material is contained in a cathode to raise electron emission characteristic.
Yet, in view of saving of a scarce resource, there has been a restriction on use of thorium as emitter material. Thus, there is a demand to avoid extensive thorium usage. In addition, since thorium is a radioactive material, there may be legal regulations restricting handling or use. In view of these circumstances, a discharge lamp having the structure, in which only the tip of cathode contains emitter material, has been variously developed. Japanese Patent Application Publication No. 2010-33825 discloses a cathode structure of a short arc type discharge lamp.
FIGS. 3A and 3B show such background art. Specifically, FIG. 3A is a view of a short arc type discharge lamp. FIG. 3B is an enlarged view of a cathode structure. As shown in FIG. 3A, an anode 11 and a cathode 12, which are made of tungsten, are arranged to face each other in an arc tube 10 of the short arc type discharge lamp 1. Light emitting material such as mercury and xenon is enclosed in the arc tube 10. In addition, although FIG. 3A shows the short arc type discharge lamp 1 in which vertical lighting is carried out, such a lamp may be used in horizontal lighting, depending on application. And the cathode structure of the lamp is shown in FIG. 3B, in which the cathode 12 comprises a cathode main body portion 12b made of tungsten having high purity, and an emitter portion 12a which is integrally formed with the cathode main body 12b. Emitter material such as thorium oxide is contained in tungsten of this emitter portion 12a. 
In such a lamp in which such thorium is used as emitter material, the thorium oxide contained in the thoriated tungsten of a tip portion of the cathode is returned when temperature of the cathode surface is raised during lamp lighting, and it turns into thorium atoms to diffuse in an outer surface of the cathode, thereby moving to a tip side where the temperature is high.
However, in the above-mentioned background art, the emitter material, which actually contributes to an improvement of the electron emission characteristic at time of lamp lighting, is limited to emitter material contained a portion from the outer surface of the tip of the cathode to a very shallow area. Although the emitter material evaporates and is consumed by heat at the outer surface of the tip of the cathode, it is expected that the emitter material is supplied to the outer surface by concentration diffusion from the inside of the cathode. However, supply thereof, which is caused by concentration diffusion from the inside of the cathode whose temperature is low, is not fully made, compared with the consumption amount in the outer surface whose temperature becomes the highest. Thus, the supply amount does not catch up with the consumption amount, and consequently, although the inside of the cathode is rich in emitter material, a phenomenon occurs in which the emitter material is dried on the cathode surface. Further, in the background art, even if such emitter material is contained inside the tip of the cathode, the emitter material is not fully utilized, when the emitter material is dried on the surface of the tip of the cathode. In turn, the electron emission characteristics deteriorate and causes a flicker.